The present invention relates to an output circuit of a stereo/monaural audio device, by which a monaural signal reproduction can be possible without interference between left and right channel signals in the stereo audio device.
In an audio device for reproduction of stereo signals, a conventional output circuit by which a monaural signal reproduction is possible by switching between a monaural circuit and a stereo circuit has a construction such as shown in FIG. 3. In FIG. 3, the output circuit includes a left channel circuit 10 and a circuit 20 which is used as a right channel circuit as well as a monaural signal circuit. Reference numerals 11 and 21 depict operational amplifiers, 12 a jack having an internal switch 13, 22 a jack having no internal switch, 14 and 24 plugs for deriving output signals, 15 a stereo/monaural changeover switch and 16 and 26 audio systems.
In stereo signal reproduction, left and right channel signals are supplied to the left channel 10 and the right channel 20, respectively. After amplified by the operational amplifiers 11 and 21, these signals appear at the jacks 12 and 22, respectively. When the plugs 14 and 24 are inserted into the the jacks 12 and 22, respectively, the internal switch 13 of the jack 12 is opened and therefore the respective signals are allowed to pass to the audio systems 16 and 26, respectively, to thereby produce stereo sound.
When the plug 14 is pulled out from the jack 12 under this condition, the internal switch 13 thereof is closed, so that an output of the operational amplifier 11 is combined with the output of the operational amplifier 21. Since, at this time, the changeover switch 15 is also actuated to the shown position, the right channel signal is output from both of the audio systems 16 and 26, simultaneously. Besides, for a monaural operation based on a monaural input signal supplied to only the right channel, it is possible to supply the monaural signal to both channels and shift phase of the monaural signal in one channel with respect to that in the other channel. The phase shifted two monaural signals are supplied to the respective sound systems to provide stereo-like sound.
When the output plug 14 of the right channel of the stereo device operating in a stereo mode is pulled out, an operation like monaural operation can be obtained and a monaural signal is derived from the output plug 24 of the left channel. In such case, however, signals on the both channels may interfere with each other at certain frequencies and/or at a certain timing, resulting in tone variation of the resultant output from the plug 24 or dropout of some portion of the signal.